I am looking forward to your reply.
或直接写成 Look forward to your reply. 这句还是比较经典的,也相对礼貌;不过假若是比较熟悉的人写成上面那位朋友的答案也可以。
Please write back to me as soon as possible.或 I hope to receive your reply as soon as possible.
望采纳~
I hope that as soon as possible given your reply
I hope you reply as soon as possible